This application relates to a process for removing contaminated sediments from the bottom of lakes, reservoirs, rivers, streams, and other water bodies, while at the same time minimizing the release of particulate to the ambient environment during the excavation process, and cleaning the waters extracted during this process.
Dredging to extract and remove bottom sediments involves mechanically raking, grabbing, penetrating, cutting, or hydraulically scouring the bottom of the waterway to dislodge sediment. This is a special problem when the dredge site contains highly contaminated materials that must be removed without dispersing the sediments and contaminating alternate locations. Bottom sediments disturbed by dredging operations, but not removed from the water body, pose several environmental problems. If the sediments are contaminated, the resuspension of particles provides the means for contaminants to migrate from their original source to new locations, impacting marine life in these areas and ultimately the ecosystem and food chain.
Of particular concern is the resuspension of fine clay and organic sediment particles (micron- and submicron-sized). Such particles tend to concentrate contaminants due to their high absorptive properties and the large surface areas that are cumulatively available in this very small size range. In addition to sediment toxicity problems, excessive particulate resuspension in environmentally sensitive areas results in visible turbidity, which may inhibit fish migration or reproductive patterns, impair fish gills, or cover larvae, eggs or bottom-feeding invertebrates. United States Environmental Protection Agency, Assessment and Remediation of Contaminated Sediments (ARCS) Program, Remediation Guidance Document, USEPA 905-B94-003, October 1994.
Current methods of dredging can be divided into two general categories. They include mechanical dredging and hydraulic dredging. The fundamental difference between these categories is in the form in which the sediments are removed. Mechanical dredges remove the sediments directly with clamshell-type buckets. The operation consists of lowering the bucket with a crane to the bottom of the waterway, scooping or extracting the sediment, and bringing the sediment to the surface for disposal (typically in a dredge barge). Hydraulic dredges, sometimes referred to as vacuum dredges, are designed to vacuum up bottom sediments. Unless the sediments are very loose, vacuum dredges require cutter heads or alternative means to dislodge the dredge material so that the sediment can be vacuumed into the dredge head.
Mechanical dredging operations typically yield much lower liquid to solid ratios (30 to 70 percent by weight) compared to hydraulic dredges (less than 1 to 10 percent by weight). Mechanical dredges, however, have the potential to resuspend sediment and contaminate the ambient water as the bucket initially contacts and penetrates the sediment. This action scoops and rakes the sediment, extruding bottom sediments perpendicular to the cut) away from the bucket. It introduces contaminated sediments into the inside of the bucket, which contacts and displaces water inside the bucket, forcing this (now contaminated) water back into the water column. When the bucket is lifted it produces an upswell that releases additional sediment particles into the water column. In addition, buckets that are not adequately sealed or unable to close completely, during the excavation process, will drain contaminated sediment particles back into the water column during the entire lifting cycle.
While hydraulic dredges provide a vacuum to draw in particulate matter during the dredging operation, cutter blade sediment agitation and raking is a major source of particulate resuspension, and vacuum recovery of these resuspended particles in conventional hydraulic dredge systems, particularly if there is any current in the waterway, is typically not very high. In addition, because hydraulic or vacuum type dredges remove and transport sediment in a slurry form, large quantities of water are collected with the sediment and must be contained and treated. This is typically accomplished by discharging the solids and water into an impoundment area to permit solids removal (typically by settling). The liquid portion of the slurry is normally discharged back into the ambient water environment after the settling process. The use of large impoundment areas for the dredging of contaminated sediments introduces the risk of on-shore or groundwater contamination and would, in general, be considered a questionable practice with highly contaminated sediments. Finally, a high degree of treatment of this impounded water is necessary to ensure that the excess water can be safely discharged back into the ambient water environment.
Some conventional mechanical dredges have been redesigned in an attempt to minimize sediment resuspension (Ouwerkerk, R. and H. Greve (1994). xe2x80x9cDevelopments in Dredges During the Last Decade.xe2x80x9d Pages 690-699 in Dredging ""94, Proceedings of the Second International Conference on Dredging and Dredged Material Placement, Edited by: E. C. McNair, Jr., American Society of Civil Engineers. 1994. Zappi, P. A. and D. F. Hayes. xe2x80x9cInnovative Technologies for Dredging Contaminated Sediments.xe2x80x9d Improvement of Operations and Maintenance Techniques Research Program, U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, Miss. Miscellaneous Paper EL-91-20. September 1991. Herbich, J. B. Handbook of Dredging Engineering, McGraw Hill, Inc., N.Y. 1992.). The primary objective of most mechanical dredge equipment modifications is to create as tight a seal as possible on the bucket through the installation of rubber backing compressible seals and sensors (proximity switches) to ensure the bucket is closed prior to lifting in the hope of minimizng spillage. While these types of systems offer improved designs, they do not eliminate the impact due to raking, upswell, and water displacement that will occur on excavation.
Numerous modifications and subcategories of hydraulic dredges have been developed to mitigate problems associated with hydraulic dredging particle resuspension. Pneumatic dredges, a subcategory of hydraulic dredges, use alternating cycles of negative and positive air pressure in a submerged chamber to draw sediment through a pipe into the chamber and to propel the sediment to the surface. Hydraulic or pneumatic dredges characterized as airlift dredges, amphibex dredges, bucket wheel dredges, clean up dredges, cutterhead dredges, delta dredges, dustpan dredges, eddy pump dredges, horizontal auger dredges, plain suction dredges, pneuma pumps, and oozer dredges are dredges that make use of hydraulic pumps and/or air compressors to draw in sediment (Cleland, J., Advances in Dredging Contaminated Sediment, Scenic Hudson, Inc., 1997). Many have specific features that attempt to reduce sediment dispersion that results from cutterblade, jetting, or raking mechanisms. None of these methods, however, significantly reduce or have suitable provisions for managing large volumes of contaminated water that are generated in the process. Large impoundment areas and suitable treatment methods are still needed to contain fine contaminant particulates that are drawn up with these sediments.
Several recent patents have proposed methods that make use of compressed air and/or hydraulic pumping operations to collect contaminated sediments in a manner that minimizes sediment dispersion.
Lynch in his U.S. patent, Contaminated Marine Sediments Dredging Apparatus, U.S. Pat. No. 5,540,005, Jul. 30, 1996, proposes the use of a dredging device that is comprised of scoop buckets, retractable rigid silt curtains, and the means to introduce pressurized air into the dredging device. Lynch""s device is intended to collect dredge matter in a manner that minimizes the extrusion of soil out the sides of the apparatus during the excavation process (the purpose of the rigid silt curtains) and minimizes the extraction of water from the interior of the apparatus (water displacement) when contaminated dredge material is scooped in by maintaining a pressurized dredging device that reduces water intrusion and hence displacement. In a subsequent patent Lynch, Extrusion Dredging Apparatus, U.S. Pat. No. 5,561,922, Oct. 8, 1996, proposes the use of a scoop bucket, similar to his earlier patent, but with mechanical modifications that includes powered helices to propel soil upward to the surface. No method for handling the liquid and sediment extracted is defined. Lynch""s device makes no attempt to collect any dispersed particulate matter outside the limits of the boundary of the scoop.
Sturdivant describes an invention, Hybrid Dredge, U.S. Pat. No. 5,311,682, May 17, 1994, that is characterized as a hybrid dredge that utilizes features of both a mechanical dredge and a hydraulic dredge in an attempt to reduce resuspension problems associated with mechanical dredging operations and large water volumes associated with hydraulic dredging operations. These objectives are similar to the subject invention, however the approach differs significantly. Sturdivant proposes to achieve these objectives through an invention that he claims can remove dredge material at low velocities without generating large volumes of water. The proposed equipment consists of a clamshell bucket attached to a rigid extension boom situated on a floating platform fixed at a position by use of adjustable spuds. The clamshell deposits the material into the hopper of a positive displacement pump that transports the sediment collected in the bucket to the disposal area, similar to a hydraulic dredge, but at a lower water to solids ratio. Sturdivant claims the particle suspension can be reduced by use of a boom that extends and contracts into the sediment instead of using a bucket controlled by a crane that is dropped into the sediment, resulting in particle resuspension problems. No method is provided to collect any dispersed particles.
It is the nature of subsurface dredging that all current dredging methods will disperse fine and sometimes coarse, particulate matter into the water column. It is apparent from the above that to effectively collect contaminated particles, suspended during the dredging process, it is necessary to collect both the particles and a given quantity of water associated with the water column or control zone into which these particles are suspended. It is also apparent that to effectively manage the water and suspended particles without the need for constructing large onshore impoundments or treatment facilities, it is necessary to control the volume of water in said water column in order to minimize said quantity. Finally, it is apparent that to treat such waters will require a system with a high degree of efficiency and which will be sufficiently mobile so that the treatment process can move with the dredging operation up and down a given waterway.
In contrast to the prior art, the invention being disclosed herein also relates to the development of individual processes that permit the extraction of contaminated sediments or dredge materials from the bottom of natural or artificial (man-made) water bodies, including but not limited to streams, lakes, reservoirs, rivers, estuaries, canals, and harbors in a manner that provides for effective cleanup, minimizes the resuspension and migration of particles of sediment collected during the process.
It is therefore an object of the invention to extract sediments from the bottom of lakes, rivers, reservoirs, and other water bodies in a manner that minimizes the dispersion of transport of particles during the dredging process.
It is also an object of the invention to utilize a mechanical dredge or bucket-type of dredge that yields a relatively low liquid to solid ratio as the means to collect the bottom sediments.
It is a further object of the invention to utilize a mechanical pressure-controlled enclosure to house the mechanical dredge to capture and contain particles dispersed during the dredging process.
It is a yet another object of the invention to treat water collected in the process with a polymeric micro or ultrafiltration membrane treatment system to enable the discharge of this treated water back into the ambient environment.
It is also a further object of the invention that such a treatment system be incorporated into a self-contained dredge water treatment vessel.
It is still a further object for the self-contained water treatment vessel to contain the necessary equipment to collect and store excess dredge water, provide pretreatment to remove high solids loadings, if necessary, pass the liquid stream through the membrane filter, recirculate and treat the retentate stream, and collect and stabilize solids collected in this process for land disposal.
It is yet a further object of the invention to provide post-treatment for removal of soluble components in the permeate stream, if necessary.
It is also another object of this invention to collect dredge water during dredging operations by direct discharge into the dredge water treatment vessel or by discharge into an intermediate storage barge available to store the dredge water until such time as treatment may be effected.
It is a further object of the invention to use polymeric membranes capable of micro and ultrafiltration as a treatment method.
It is a further object of the invention to use either positively pressurized or vacuum driven hollow fiber membrane treatment systems to treat the contaminated water.
It is a further object of the invention to provide such treatment system on-board a self-contained treatment vessel.
It is also an object for this self contained water treatment vessel to contain the necessary equipment and treatment operation to pump the contaminated water, recirculate and treat the retentate stream, collect, stabilize, and treat solids and organisms removed from the contaminated water, and clean and maintain the filters used in the process.
It is yet another object of the invention to discharge treated water directly back into the ambient water environment.
It is a further object of the invention to convert the collected solids generated in the treatment process system to a stabilized form prior to disposal.
It is also an object of the present invention to improve over the disadvantages of the prior art.
In keeping with these and other objects of the invention, which will become apparent, a system is described for collecting sediments, particularly those that contain contaminants that may be harmfull to the aquatic environment if released and dispersed during the dredging process. Subsequent treatment is provided in a self-contained dredge water treatment vessel using membrane filtration to filter and remove such particles prior to release of the treated water back into the ambient environment.
The subject invention provides a new method to collect sediments with low liquid to solids ratios using an excavator, such as, mechanical bucket, with a dispersed particulate containment system and a dispersed particulate treatment system, capable of both containing and removing collecting sediment particles disturbed and suspended into the water column during the excavation process.
In this excavator embodiment, the system provides for the containment of particles by use of a specially designed enclosure that envelops and isolates a mechanical dredge (such as a hinged clamshell bucket) and introduces an alternating cycle of positive and negative pressure within the enclosure to control water intake and collect sediment particles that are dispersed during the dredging process. Dispersed sediment particles collected in this process are diverted to a water treatment system, specially designed to remove suspended solids captured within the enclosure, so that the collected water can be safely discharged to the ambient environment.
Operation of this excavator embodiment of the sediment collection system involves the lowering of the enclosed excavator from a vessel or from the shore (if it is convenient to do so) to the location in the water body where sediment extraction is desired, and retrieving the sediment and any dispersed particles generated during the process. The collected sediment is discharged to a dredge barge or other suitable receptacle and the dispersed particles are transported to the water treatment system for removal. Operation of the excavator enclosure involves two cycles: 1) a submersion cycle in which the enclosure is positively pressurized to prevent liquid from entering the enclosure, and 2) a recovery cycle in which the enclosure is negatively pressurized and water is drawn into and pumped from the enclosure to collect particles suspended during the excavation process.
While water collected in the enclosure may be pumped to any suitable treatment system for the removal of particulate matter and soluble contaminants if necessary, the inventors have included in this invention a preferred embodiment for a micro- or ultrafiltration membrane treatment system for micron and submicron sized particulate removal. Such a system has not been used for the treatment of effluents from dredging operations and the capacity of such system to remove extremely fine (submicron-sized) particles makes it well suited for such a process application. The treatment system of this micro- or ultrafiltration membrane treatment embodiment includes two primary stages for treatment of the particle-laden water prior to disposal. The first stage includes a flow equalization stage. This stage, which is intended to equalize the flow to the second or membrane filtration stage of the process, is also available for pretreatment, if necessary, for preliminary solids removal and oily water separation. The second stage includes the membrane system. This stage is designed to remove very fine micron and submicron-sized particles remaining in the water. If needed, post-treatment could be included to remove residual soluble contaminants in the effluent stream using processes such as activated carbon adsorption or chemical oxidation processes.
Commercially available membrane filters act as barriers, that can be used to separate particulate and dissolved components in both liquid and gaseous streams. The particular focus of the treatment proposed in this invention is separation of particles in the microfiltration (xcx9c0.1 to 100 microns) and the ultrafiltration (xcx9c0.01 to 0.1 micron) range.
There are presently two major types of commercially available microfiltration and ultrafiltration systems. These are polymeric and ceramic systems. These two systems are typically categorized, with respect to the supporting membrane module or configuration, as hollow fiber polymerics, tubular polymerics, spiral wound polymerics, plate polymerics, tubular ceramics, dense-pack ceramics. Other membrane configurations or modifications of existing configurations, such as flat membrane systems, rotary cylinders, and rotating disc modules are also in use and new configurations are in continual development. The inventors, having tested both ceramic and polymeric systems, have found polymeric systems to be the preferred system from a cost and effectiveness viewpoint for the subject application.
Most microfiltration and ultrafiltration systems in operation at the present time utilize positive pressure as a driving force to pass liquids through the membrane. Pressurized systems can be operated in a dead-end mode, in a cross-flow mode or in a dynamic filtration mode. In a dead-end mode, all of the incoming raw water, with the exception of a relatively small retentate or blowdown stream, is passed through the membrane. In a cross-flow mode the feed is pumped tangentially to the membrane surface. In such an operation, one stream enters a membrane module (the feed stream) and two streams exit (a relatively large recirculation stream and the permeate stream). The cross-flow stream induces a shearing and hence cleaning action on the membrane, which leads to reduced particulate fouling or caking and a higher average flux rate than a dead end operation. Dead-end pressurized systems typically operate under lower transmembrane pressures than cross-flow systems. A dead-end or cross-flow pressurized membrane system is suitable for use in the subject invention. In a dynamic microfiltration operation, the filter medium is moved (typically rotated) at sufficient speed relative to the fluid stream to produce sufficient shear forces to maintain a free filter surface for extended periods of time. A dynamic microfiltration membrane system is suitable for use in the subject invention.
In recent years, immersed or submerged vacuum-driven hollow fiber membranes have been introduced commercially. An immersed hollow fiber membrane filter operating under negative pressure is a polymeric filter that achieves filtration by drawing water through a thin fiber (membrane) surface into the hollow annular inner core of the fiber. Permeated water is transported through the inner core to a common header pipe where it is discharged. To improve flux rates and reduce fouling, compressed air is typically introduced at the bottom of a fiber module to agitate the fibers and induce tangential shear forces adjacent to the membrane, thereby preventing solids buildup on the individual fibers. An immersed hollow fiber membrane system is suitable for use in the subject invention.
In both positively pressurized and vacuum driven membrane systems a gradual solids buildup will occur in the recirculation or retentate stream. Periodic blowdown or treatment of pressurized system retentate or immersed filter reactor streams is necessary as part of the operational process to limit the influent solids concentrations to the filters. In both systems backpulsing and interim cleaning of filters will be necessary. Chemical treatments are available (oxidizing agents, alkalis and acids) to periodically backpulse and clean the membranes to restore their flux rates should fouling occur.
To implement a membrane filtration strategy for the referenced applications, the membrane filtration embodiment of the subject invention provides for the use of a xe2x80x9cself-contained water treatment vessel,xe2x80x9d designed with water storage and equalization capacity, solids treatment and handling, and soluble organic treatment, if needed.
In dredge water treatment operations, the treatment vessel is located directly adjacent to the dredging operation to permit the discharge of excess water into the treatment vessel or into a storage barge of sufficient capacity to store excess dredge waters, if needed, prior to discharge of the water to the treatment vessel. While one focus of such a treatment vessel is to treat dredge waters generated during excavation operations with the aforementioned excavator, such a treatment vessel can also be used to collect and treat waters from hydraulic dredging operations. In hydraulic dredging operations, where the sediment is extracted by vacuum generating large quantities of water, a settling barge or vessel to remove the sediment, would in most cases, be required with overflow from the barge directed to either a storage vessel or the membrane treatment vessel.